1. Field of the Invention
The present invention relates to a pushbutton switch key sheet for use in various electronic apparatuses, such as a portable information terminal, including: a mobile phone; a personal digital assistant (PDA), a vehicle-mounted AV apparatus, a remote controller, and a personal computer.
2. Description of the Related Art
Many pushbutton switches for various electronic apparatuses, such as a mobile phone and an AV apparatus, are of a construction in which the pushbuttons (i.e., key tops) for performing input operation through depression are exposed through an operation opening formed in the casing of the electronic apparatus. To be specific, it is common practice to place a key sheet having key tops on a board on which contact switches are arranged and to cover the front side of the key sheet with the casing to thereby incorporate the key sheet into the casing.
Electronic apparatuses, whose functions are becoming more and more sophisticated, adopt a construction in which the heat generated within the apparatus is dissipated. This heat is generated by the mounted components, such as semiconductor devices and electronic components, mounted on the board with high density. Above all, the semiconductor devices increase in heat generation amount in accordance with an improvement in processing capacity and an increase in processing capacitance, and if local heat storage is left unattended, there is a fear of a malfunction and failure. Thus, it is necessary for the heat generated around the mounted components to be effectively dissipated to the ambient space without being allowed to locally stay.
As a conventional example of a countermeasure against this heat, a cooling component, such as a heat sink or a cooling fin, is attached to the heat-generating mounted component through the intermediation of a heat conductive sheet, a heat conductive grease, etc. However, while such a countermeasure against heat is taken for the mounting surface side of the board, no sufficient countermeasure against heat has been taken for the back surface side thereof. Thus, as the heat generation amount increases, local heat storage occurs also on the back surface side of the board.
For example, in a portable electronic apparatus, such as a mobile phone, such local heat storage is a serious problem that should be solved as soon as possible. That is, in a portable electronic apparatus, there are mounted many functions involving a large processing load, such as a moving picture reproducing function. Thus, ideally, it is desirable to take such a countermeasure against heat as mentioned above for both sides of the board. However, in view of the fact that there is a demand for a further reduction in the thickness of electronic apparatuses, it is rather difficult to secure the requisite arrangement space for a cooling component, etc. between the key sheet and the board. Further, the fact that the key sheet is a movable component that is movable through depressing operation of the key tops also constitutes a factor making it difficult to cope with the heat between the key sheet and the board.
In this connection, for example, JP 2000-311050 A proposes a countermeasure against heat using a radiation electromagnetic wave absorption shielding plate formed of metal provided between a board contained in a keyboard and key tops for input operation, and a graphite sheet attached to this shielding plate. However, as stated above, in a portable electronic apparatus, in particular, of which a further reduction in thickness is required, there is no room left between the board and the key sheet for such a large gap as would allow taking a countermeasure against heat.